The present invention is directed to inductive proximity sensors in general, and more particularly, to an inductive proximity sensor with induction coils in fixed relationship with an inductive core by compression to avoid: (1) the use of adhesives to secure the induction coils to the inductive core; and (2) adjustment or calibration to achieve the desired inductive output, and method of assembling the same.
Inductive proximity sensors typically comprise a core, which may be “C” or Omega shaped, for example, fabricated from a highly-permeable metal, with two inductive coils on bobbins placed over each leg of the core. The two coils are typically wound around their respective bobbins in opposite directions (one wound clockwise and the other wound counter-clockwise) and electrically connected in series. The series connected coils of the sensor are generally driven by an AC voltage at a desired frequency. The generated coil current, which may be monitored by a current sensing device, is commonly used as an inductive output of the sensor. Generally, the inductive output changes value when a target to be sensed moves from a near to a far position with respect to a sensing face of the sensor, and vice versa. There should be a sufficient change in value of inductive output over the span of operating conditions in order to be able to distinguish between the near and far target positions.
The inductive output of the sensor is determined by several factors, including core material, core geometry, number of turns of coil wire, coil (bobbin) geometry, operating frequency and voltage, coil resistance, sensor housing material, and the relative position of the coils to the core, for example. The significant characteristics of all of the materials used in the sensor as well as the assembly process is controlled so that the assembled sensor may exhibit an established standard inductance (within tolerances). Usually, in order to meet an inductive specification, the sensor assembly or transducer is “calibrated”. The calibration may be accomplished in several ways. Three of the most common methods are: 1) move one of the coils along its core leg until the desired inductance is achieved and then secure the bobbin to the core leg with an adhesive, which may be an epoxy, for example; 2) add or remove turns of wire from one or both of the coils until the desired inductance is achieved; and 3) add an adjustable permeable shunt to the assembly which will magnetically interact with the core thereby effecting an adjustment to the transducer inductance. All of these calibration methods include manual intervention by the assembler of the sensor. It would be advantageous, from a manufacturing perspective, to successfully assemble the sensor without the need for calibration.
As noted above, it is currently common practice during calibration to secure the bobbin (on which the coil is wound) to the core leg using an epoxy adhesive. Eliminating this step from the assembly process would significantly increase the reliability of the sensor and reduce manufacturing costs.
The present invention as will be described in greater detail herein below incorporates features including self aligning coils which will allow for the successful assembly of a proximity sensor without calibration, and thus, without the use of an adhesive to secure the coil bobbin to the leg of the core during assembly of the sensor.